Addressing the ocean methane paradox: the role of microenvironments in oceanic methane production

为什么世界上层海洋的甲烷会过饱和？我们知道这是事实，但不明白为什么。有证据表明，一部分甲烷来自含氧水域的原位生产，然而，这似乎与我们所知的甲烷生成--严格意义上的厌氧过程--相矛盾。这种现象被称为“海洋甲烷悖论”。然而，如果在上层海洋中存在厌氧微生物场所，那么它们完全有可能在其中发生产甲烷作用。我们现在认为，海洋浮游动物、它们排泄的粪便物质和其他沉淀物可能为远洋水域提供了这些厌氧微生物场所。我们在SAMS的研究小组进行的工作支持这一假设。我们现在已经清楚地确定了海洋浮游动物粪便颗粒和沉淀物中存在产甲烷微生物。这一点，再加上数据显示，甲烷浓度升高与这些地点有关，这使得人们对这种厌氧过程如何在远洋水域活跃地发生有了更深入的了解。我们还知道，产甲烷菌可以使用一系列底物，包括二氧化碳和甲酸盐。然而，我们从浮游动物粪便颗粒中研究的一些产甲烷菌属于甲烷杆菌属，并被认为利用一碳(C1)化合物，包括二甲基硫化物(DMS)和甲胺(Mas)。这两种化合物的潜在来源是二甲基磺酸碘丙酸酯(DMSP)和甘氨酸甜菜碱(GB)，它们是由海洋浮游植物产生的，以维持它们在海水中的渗透平衡。很可能，当浮游动物吃浮游植物时，它们至少会消耗一部分DMSP或GB，然后将其包装到它们的粪便颗粒中。DMSP和GB被认为是通过微生物活动分别转化为DMS和MAS的。因此，放牧代表了这些C1-化合物进入浮游动物肠道和粪便颗粒的途径，在那里它们可能是产甲烷的底物。认为DMS或MAS产生的气溶胶粒子可能通过影响云的反照率和太阳辐射的反射而对自然降水的pH值做出贡献，并在气候控制中发挥作用。因此，浮游动物粪便颗粒可能是产生温室气体和消除气候反馈气体的重要场所，对我们理解和模拟海洋在气候变化中的作用具有重要影响。我们建议进行一个多学科项目，以加深我们对浮游动物、它们的粪便颗粒和沉淀物作为水柱中潜在的原位产甲烷地点的作用的了解。我们的主要目的是阐明藻类衍生化合物在产甲烷中的作用，确定合养关系在这一过程中的重要性，并调查这些地点内替代底物的使用。这应该使我们能够确定负责这一过程的主要产甲烷基团，以及它们如何受到环境和其他微生物的影响。SAMS和其他机构的工作组已经证明了这项工作的先决条件。然而，这项工作中的大部分虽然令人兴奋，但都是初步的，其过程仍然知之甚少。研究将使用最先进的技术(包括实时聚合酶链式反应、稳定同位素探测、稳定同位素质谱仪、CARD-FISH)和已建立的分析和微生物方法(培养和培养无关)进行。此外，通过合作学生的工作，我们希望为这项工作增加令人兴奋的新方面，包括进一步鉴定分离的产甲烷菌，并使用卡片-FISH和共聚焦显微镜进一步了解它们的位置。通过将这些研究领域与新的方法相结合，我们希望开始解开海洋甲烷悖论。

Why is the world's upper ocean supersaturated with methane? We know that it is, but do not understand why. Evidence shows that a portion of the methane comes from in situ production in oxygenated waters, however that seems to contradict all we know about methanogenesis; a strictly anaerobic process. This phenomenon has been termed the 'oceanic methane paradox'. If, however, there were anaerobic microsites in the upper ocean, then it is entirely possible that methanogenesis could occur within them. We now think that marine zooplankton, their excreted faecal material and other sedimenting particles may provide these anaerobic microsites in pelagic waters. Work conducted by our research group at SAMS supports this hypothesis. We have now clearly identified the presence of methanogens (methane producing microorganisms) within marine zooplankton faecal pellets and sedimenting particles. This, along with data showing that elevated methane concentrations are associated with these sites, has led to greater insights into how this anaerobic process may be actively occurring in pelagic waters. We also know that methanogens can use a range of substrates, including carbon dioxide and formate. However, some of the methanogens we have studied from zooplankton faecal pellets are affiliated with the genus Methanolobus, and are thought to utilize one-carbon (C1)-compounds, including dimethylsulphide (DMS) and methylamines (MAs). Potential sources of these two compounds are dimethylsulphoniopropionate (DMSP) and glycine betaine (GB), which are produced by marine phytoplankton to maintain their osmotic balance in seawater. It is likely that when zooplankton eat phytoplankton they consume at least some of the DMSP or GB, which is then packaged into their faecal pellets. DMSP and GB are thought to be converted into DMS and MAs respectively by microbial activity. Grazing therefore represents a pathway for these C1-compounds to enter into the zooplankton gut and faecal pellets, where they may be substrates for methanogenesis. It is thought that aerosol particles generated from either DMS or MAs may contribute to the pH of natural precipitation and play a role in climate control due to their influence on cloud albedo and reflection of solar radiation. Therefore, zooplankton faecal pellets could be instrumental sites both in the production of a greenhouse gas and the removal of climatic feedback gases, having important consequences for our understanding and modelling of the role the oceans play in climate change. We propose to conduct a multidisciplinary project that will further our understanding of the role of zooplankton, their faecal pellets and sedimenting particles as potential sites of in situ methanogenesis in the water column. Our main purpose is to clarify the role of algal derived compounds in methanogenesis, determine the importance of syntrophic relationships in this process and investigate the use of alternative substrates within these sites. This should enable us to determine the main methanogenic groups responsible for this process and how they are influenced by their environment and other microorganisms. The prerequisites for this work have been demonstrated by the group at SAMS and others. However, much of this work, though exciting, is preliminary and the processes remains poorly understood. Research will be carried out using both state of the art techniques (including real-time PCR, stable isotope probing, stable isotope mass spectrometry, CARD-FISH) and established analytical and microbiological methods (culture & culture independent). In addition, through the work of a tied studentship, we hope to add exciting new aspects to this work including further characterisation of isolated methanogens and an increased understanding of their location using CARD-FISH and confocal microscopy. By combining these areas of research with new methodology we hope to start to unravel the ocean methane paradox.